This invention relates to an improved heated fluidized bed reactor and a method of using such reactor. In a preferred embodiment the improved heated fluidized bed reactor is useful in the production of polycrystalline silicon by the pyrolysis of silane containing gases.
A variety of means are well known in the art for supplying the necssary heat to fluidized bed reaction zones. A suitable heat transfer fluid and inductive or electrical resistance heaters are examples of direct means for supplying heat to the exteriors of conventional fluidized bed reactors. While adequate for the purposes of some fluidized bed applications, supplying heat to the exteriors of fluidized bed reactors is not entirely satisfactory for other fluidized bed applications because of the particular configurations and reactions occurring therein, particularly those reactions or operations where large amounts of heat are required. Undesirable side effects may result due to inefficient and/or insufficient introduction of heat into the fluidized bed reaction zone using conventional means of supplying heat to fluidized bed reactors.
The production of polycrystalline silicon from silane containing gases in a fluidized bed reaction zone is an example of the limitation of conventional means for heating fluidized beds. In this example, silicon particles are suspended in a fluidizing gas stream into which silane is injected. The process conditions are maintained so that the thermal decomposition of the silane occurs heterogeneously, i.e., the silane decomposes on the surface of the particles in the fluidized bed. The particles grow and enlarge due to the deposit of silicon thereon. Sufficiently large particles of silicon product are grown to allow for conventional removal of the product from a collection zone below the reaction zone. The byproduct hydrogen and other gases are separately removed as overhead gas from the reaction zone.
In the case of a polycrystalline silicon process, conventional means of supplying heat to the outer walls of a fluidized bed reaction zone result in an undesired coating of silicon on the wall of the reaction zone, possibly in preference to the desired deposition of the silicon onto the fluidized particles. The deposition of silicon on the reactor walls is partially a result of the elevated temperature of the reactor wall compared to the cooler temperature of the silicon particles. The deposition of silicon on the reactor walls has the effect of reducing the heat transfer efficiency into the reaction zone as well as the overall process efficiency.
It is known that in the pyrolysis of silane containing gas, heat may be supplied by capacitive heating of the fluidized bed reaction zone, as discussed in U.S. Pat. No. 4,292,344 to McHale. Other methods of heating, such as uniform induction coils, electrical resistance elements, and indirect gas fired heaters, have also been utilized on the outer walls of fluidized bed reactors and are disclosed in U.S. Pat. Nos. 3,012,861 to Ling and 3,012,862 to Bertrand, et al.
The present invention provides an improved heated fluidized bed reactor wherein heat may more effectively be introduced into the reaction zone. The reactor has widespread applications in fluidized bed operations requiring the input of heat. Exemplary of such operation is the production of high purity polycrystalline silicon.